I believe this motherboard has the same configuration for storage as the B550-E I had before. If this is correct, top M.2 slot is capable of Gen 4 x4*, and bottom one is Gen 3 x4 - using both simultaneously knocks out two of the SATA ports, so if you have all SATA ports used, you cannot use the bottom M.2 slot. This is because the B550 chipset doesn't have enough bandwidth.
Gen 4 NVMe is only supported on the top slot if you are using a Matisse (3000) or Vermeer (5000) processor. Renoir (4000G) and Cezanne (5000G) are not Gen 4 capable, so this slot will be limited to Gen 3 and available bandwidth may be further reduced if the integrated graphics are enabled and discrete graphics or lots of PCIe expansion cards are installed.
Otherwise, B550-F's only other differences are that it does not come with a Wi-Fi module standard (unless you buy the Wi-Fi variant of the -F II refresh), significantly weaker VRM and absence of debug code reader, the area where it's located on the model F is vacant.
If your motherboard does not have two slots, then it is not a B550-F.